A semi-analytical method is proposed in this study to estimate tube wall temperature versus time for Waste Heat Recovery Units (WHRU) and fired heaters in a typical LNG plant. The results are used to estimate Safety Time to avoid failure of tubes. Safety time estimation is crucial for control loop design of equipment in the operating plant. Two case studies are presented based on real control loop design data for an LNG plant. The results show that for both case studies, process safety time for control loop is less than the tubes safety time. Therefore the control system is capable of safely stopping the equipment from operation without tube failure or degrading of the process fluid.

The proposed semi-analytical method is capable of calculating the Safety Time for WHRUs and fired heaters in an economical way while the method offers a reasonable accuracy due to its analytical nature. The proposed method can be used in Fundamental Engineering and Evaluation Designs (FEEDs), Hazard and Operability studies (HAZOPs) and Hazard Identification (HAZID) studies for LNG plants and power plants.

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